The present invention relates to the peroxidation of secondary carbon in alkanes, including alkyl groups attached to aromatic rings, and cycloalkanes using a hydroperoxide initiator.
It is well known that the various tertiary and secondary organic hydroperoxides are suitable as oxidants for a number of highly selective partial oxidations. Molecular oxygen is usually not very suitable for these oxidations since a selective or partial oxidation requires, in a stoichiometric sense, the transfer of one oxygen atom only. For example, organic hydroperoxides are particularly effective for the synthesis of alcohols, aldehydes, ketones, epoxides, cyanates and oximes.
Typical industrial processes for oxidizing alkanes and cycloalkanes involve oxidation in the presence of a catalyst such as a cobalt-naphthanate catalyst. The peroxide which is formed is only transitory and is subject to decomposition in the presence of the catalyst. The primary products are the corresponding alcohol and ketone. For example, the most widely used oxidation process for cyclohexane has only about 4 to 5% conversion of the cyclohexane and the product is about 75% cyclohexanol and cyclohexanone with the remainder being various by-products. At higher conversion levels, the selectivity of products decreases because the cyclohexanone is readily oxidized to by-products. The solution is to minimize the formation of the cyclohexanone by keeping the conversion of cyclohexane at a low level or preventing the formation of cyclohexanone from cyclohexanol such as by esterification. None of these processes produce any significant quantity of the peroxide, cyclohexyl hydroperoxide, in the final product. Typically the amount is only 5 to 10% of the product.
The main objective of peroxidations is the production of the corresponding hydroperoxides. However, the decomposition of hydroperoxides is required to maintain the free radical type chain reaction. This represents a loss in selectivity for the desired hydroperoxide. In addition, the alcohols or ketones formed from the decomposition of secondary hydroperoxides are more readily oxidized than the corresponding hydrocarbons. This over-oxidation leads to the rupture of carbon-carbon bonds and to the formation of carboxylic acids, which also facilitate the decomposition of the hydroperoxides. The result is a rapid decrease in the selectivity of products with increasing conversion.
To avoid this yield loss, the oxidation/peroxidation of the secondary carbon in various alkanes and cycloalkanes has to be carried out without the usual transition metal ions, which catalyze the decomposition of hydroperoxides. Also, the reactions have to be stopped at relatively low conversion levels. The commercial use of secondary carbon oxidations is limited by these two problems. First, in a non-catalytic oxidation, the initiation of the chain reaction must rely on the thermal decomposition of some hydroperoxide. This means that at low temperature the reaction is very slow while at high temperature the selectivity is low. Secondly, to produce the hydroperoxide of the secondary carbon at high selectivity, one can not rely on its decomposition for supplying the free radicals.
According to the invention described and claimed in U.S. Pat. No. 5,220,075 dated Jun. 15, 1993, a secondary carbon atom in an alkane, including alkyl groups attached to aromatic rings, or cycloalkane hydrocarbon is oxidized by molecular oxygen to the corresponding hydroperoxide in the absence of a catalyst. The oxidation is carried out in the presence of a tertiary hydroperoxide, which provides the free radicals needed to maintain the reaction. This method of reaction is particularly applicable to the peroxidation of cyclohexane to produce cyclohexyl hydroperoxide at a temperature of 100.degree. to 200.degree. C. and preferably 130.degree. to 160.degree. C. and a pressure of 700 to 1200 kPa using 0.5 to 10% and preferably 1 to 5% tertiary butyl hydroperoxide or tertiary amyl hydroperoxide. All percentages are expressed as mole percent unless otherwise noted. This method for the oxidation of the secondary carbon in alkanes and cycloalkanes results in a relatively high conversion and selectivity for the hydroperoxide.
Among the hydrocarbons which are commercially oxidized, cyclohexane and ethyl-benzene are industrially the most important and are used for the production of cyclohexyl hydroperoxide and ethylbenzyl-hydroperoxide. These are the key intermediates in the coproduction of propylene-oxide with styrene or with cyclohexanol. The propylene oxide, styrene and cyclohexanol are key components for the production of polymers such as polyurethane, polystyrene and nylon.
Due to the cost of the plant for producing the tertiary hydroperoxide initiators (i.e. tertiary butyl hydroperoxide and tertiary amyl hydroperoxide), the producers of ethylbenzene hydroperoxide and cyclohexane hydroperoxide would like to minimize the amount of initiator consumed per unit of secondary hydroperoxide that is produced. On the other hand, the effectiveness of the tertiary hydroperoxide for initiating the free radical type chain reaction and for achieving the optimum selectivity of the secondary hydroperoxide increases with the concentration of the tertiary hydroperoxide. An increase in the concentration of the initiator, tertiary hydroperoxide, results in a faster reaction and in an increased selectivity of products. This means lower capital and operating costs for the producer.
The effluent from the initiated peroxidation of a secondary hydrocarbon contains the secondary hydroperoxide product, the unreacted hydrocarbon and some alcohol. It will also contain considerable quantities of the tertiary hydroperoxide initiator, particularly, if the concentration of the initiator in the peroxidation reaction is maintained at a high level to improve the performance of the reaction.